Thermophotovoltaic generators using selective metallic emitters

摘要

In the literature to date on thermophotovoltaic (TPV) generators, two types of infrared emitters have been emphasized: gray body emitters and rare earth oxide selective emitters. The gray body emitter is defined as an emitter with a spectral emissivity independent of wavelength whereas the rare earth oxide selective emitter is idealized as a delta function emitter with a high emissivity at a select wavelength and a near zero emissivity at all other wavelengths. Herein, we describe an alternative type of selective emitter, a selective metallic emitter. These metallic emitters are characterized by a spectral emissivity curve wherein the emissivity monotonically increases with shorter infrared wavelengths. Tungsten typifies this class of selective metallic emitters. In a thermophotovoltaic generator, a photovoltaic cell typically converts infrared radiation to electricity out to some cut-off wavelength. The problem with gray body emitters is that they emit at all wavelengths. Therefore, a large fraction of the energy emitted will be outside of the response band of the TPV cell. The argument for the selective emitter is that, ideally, all the emitted energy can be in the cells response band. Unfortunately, rare earth oxide emitters are not ideal. In order to suppress the emissivity toward zero away from the select wavelength, the use of thin fibers is necessary. This leads to a fragile emitter typical of a lantern mantle. Even given a thin Er-YAG emitter, the measured emissivity at the select wavelength of 1.5 microns has been reported to be 0.6 while the off wavelength background emissivity falls to only 0.2 at 5 microns. This gives a selectivity ratio of only 3. Another problem with a delta function selective emitter is its low power density at practical temperatures because of its narrow emission bandwidth. The concept of selectivity can be generalized by noting that we simply wish to maximize the ratio of in-cell-band power to out-of-cell-band power. Using this generalized selectivity concept and assuming a GaSb cell covered by a simple dielectric filter, we note that the emissivity selectivity ratio for tungsten is 0.3 (at 1.5 microns) /0.07 (at 5 microns) = 4.3. In this paper, we note that the selective metallic emitters can be valuable in both radioisotope TPV generators in space and in hydrocarbon fired TPV generators here on earth.